How to Treat Powdery Mildew on Rosemary Leaves

A warm kitchen may not be the best bet for winter rosemary.

Rosemary (Rosemarinus officinalis) is as attractive in the kitchen as it is aromatic and tasty, but take care when you bring one indoors for the winter or it can fall victim to powdery mildew. Reliably winter hardy in U.S. Department of Agriculture hardiness zones 9 and up, many gardeners -- even those not in cooler zones -- pot up rosemary to have close at hand for winter cooking. Though indoor temperatures protect rosemary from freezing, the moderate temperatures, lower light and lack of air circulation can leave the plant's foliage coated with patches of unsightly white fungus you definitely don't want to add to the roast chicken. Improving cultural conditions prevents powdery mildew, but the condition can be safely treated as well.

1

Examine your rosemary plant for symptoms of powdery mildew. These are patches of dusty white spores on the surfaces of living leaves -- the needlelike foliage of the rosemary -- and woody stems. Some affected foliage may turn brown and die.

2

Clean the blades of a pair of clippers with a rag dipped in rubbing alcohol to remove any traces of disease or fungi from plants you previously trimmed with the tool.

3

Clip away the affected foliage or stem of the rosemary flush with the branch. Discard the clippings. Clean the clippers with rubbing alcohol and let them dry before putting them away.

4

Mix a solution of 1 part milk to 5 parts water in a clean spray bottle.

5

Place the rosemary plant in an area that receives bright light. Spray a plant with mild powdery mildew with the mixture. In bright light, the milk acts as a fungicide to kill any lingering powdery mildew spores. Let the plant dry.

6

Move the plant to a cool, bright area, like an unheated garage with windows or a walkout basement. Water from overhead, as this can wash powdery mildew spores off the plant, but allow it to dry. A fan, even a ceiling fan, can increase air circulation.

7

Spray plants with moderate to heavy powdery mildew with neem oil. Neem oil is a nontoxic plant-based oil that also works to prevent the development of powdery mildew. Ensure the oil covers all affected areas -- top and bottom of the foliage -- as it only works on contact. Though it shouldn't be a problem in the winter, don't apply when the temperature is more than 90 degrees Fahrenheit.

About the Author

Patricia Hamilton Reed has written professionally since 1987. Reed was editor of the "Grand Ledge Independent" weekly newspaper and a Capitol Hill reporter for the national newsletter "Corporate & Foundation Grants Alert." She has a Bachelor of Arts in journalism from Michigan State University, is an avid gardener and volunteers at her local botanical garden.